Document scanners feed and transport paper documents past one or more imaging subsystems in order to create digital image files representative of the originals. When two or more documents or pieces of paper have inadvertently been delivered to the imaging portion of the scanner by the feeding mechanism (referred to herein as a “multifeed”) there is loss of information capture because of the overlap of the documents. This leads to the need to sort and rescan those documents and a loss of productivity. Most document scanners in the commercial arena utilize ultrasonic energy transmitted through the document to a receiver to detect when multifeeds occur. This technology is also employed in other paper transport devices when knowledge about whether more than one layer of paper is present is important, such as in ATM machines that dispense paper money. Most systems rely on a substantial drop in received amplitude of the ultrasonic energy due to destructive interference of the ultrasonic energy within the thin air gap or gaps between the multiple sheets of paper. Other systems use a combination of amplitude drops and the phase shift differential of multiple sheets vs. one sheet for detection of multifeed conditions as described in the U.S. Patents listed above.
Additionally, systems have been described that detect excessive or unique sound energy using an audio frequency microphone, said energy created by the document being transported when the document or documents are being damaged, wrinkled, torn or otherwise deformed by the feeding and transport process (referred to herein as a “misfeed”). These sounds are differentiated from the normal sounds of the mechanisms via processing of the audio frequency sounds. The sounds are quantified, compared to a threshold (which may be adjustable), and then used to immediately stop the feeding and/or transport mechanism in order to prevent or substantially limit damage to the documents.
Incorporating both a receiving device or devices for the ultrasonic energy (typically in the range of 40 KHz. to 300 KHz.) and an additional device or devices for receiving audio information (typically in the range of 1 KHz. to 10 KHz.) represents both a cost penalty and a packaging challenge given the position of drive rollers and other sensors within the document transport design.